(a) Field of the Invention
The present invention relates to a method for catalytic cracking of petroleum hydrocarbons which is suitable for petroleum refining. It relates particularly to a method suitable for catalytic cracking of petroleum hydrocarbons having relatively high boiling points, for example, hydrocarbon distillates having higher boiling points than those of naphtha distillates, such as naphtha, kerosine, gas oil and atmospheric residue. The method is characterized in the catalysts used therein, which reduce excessive cracking causing the generation of cracked gases, have high selectivity toward middle distillates of higher value, and lose little activity by the steaming during regeneration. The method therefore enables efficient production of various useful hydrocarbon distillates by the catalytic cracking of various petroleum hydrocarbons, and is particularly suitable for efficient catalytic cracking of vacuum gas oils or atmospheric residue to high yields of fuel gasoline distillates (FG), which are useful as automotive fuels, or light cycle oils (LCO), which are useful as mixing components for diesel oils or heavy oils.
Catalytic cracking of petroleum hydrocarbons by fluidized-bed processes or the like is an important technique for petroleum refining. Various processes are employed for the catalytic cracking of petroleum hydrocarbons, and the feed materials and the objective products vary with processes. The most important technique is for the efficient and selective production of high yields of middle distillates of higher value from hydrocarbon distillates having relatively high boiling points, while inhibiting coking or the generation of cracked gases due to excessive cracking. Examples of such a technique are FCC processes for the efficient catalytic cracking of vacuum gas oils or atmospheric residue to a high yield of fuel gasoline distillates (FG) useful as automotive fuels or light cycle oils (LCO) useful as mixing components for diesel oils or heavy oils.
In catalytic cracking of petroleum hydrocarbons, such as the FCC processes, zeolite catalysts have been conventionally used. Zeolite catalysts however have shortcomings of poor hydrothermal resistance, and on contact with steam during catalytic cracking or during steaming at high temperatures for regeneration, their structures are destroyed, and their catalytic activity (conversion) is reduced.
Such a defect of zeolite catalysts is well known and is also apparent from the following examples.
(1) In Sekiyu Gakkaishi, 26, 19 (1983), a considerable reduction in the crystallinity of proton-Y-type zeolites (REY) induced by steaming the zeolites with 100% steam is reported. As a concrete example, it is reported that when a fresh zeolite had a crystallinity of 100, the crystallinity was reduced to 91 by steaming at 730.degree. C. for 6 hours, and to 67 by steaming at 760.degree. C. for 6 hours. PA0 (2) In Ind. Eng. Chem. Prod. Res. Dev., 16, 285 (1977), a considerable reduction in the conversion of gas oils induced by zeolites (e.g. Y-type zeolites) steamed with 100% steam is reported. As a concrete example, it is reported that the conversion is reduced to 78% by steaming at 760.degree. C. for one hour, and to 72% by steaming at the same temperature for 12 hours.
As far as the known results teach, not only the above-described zeolites but also every type of zeolite, such as various high-silica zeolites, including ZSM-5 zeolites, zeolite A, zeolite X, zeolite Y and mordenite, have insufficient hydrothermal resistance and are apt to lose catalytic activity on exposure to the atmosphere of steam at high temperatures during catalytic reactions or during regeneration, and this is a serious problem not only in the catalytic cracking of petroleum hydrocarbons but also in any process using zeolites.
Zeolites involve another defect that they induce deep cracking (excessive cracking) easily when catalytic cracking of petroleum hydrocarbons is carried out in a highly active condition. Practically, when vacuum gas oils or atmospheric residue are catalytically cracked according to the MAT evaluation method by using a commercial zeolite catalyst (MRZ-204 produced by Shokubai Kasei Kogyo Kabushiki Kaisha), the results are unsatisfactory. That is, the yields of cracked gases and coke are high, and the yield of middle distillates of higher value are low. Middle distillates are liquid hydrocarbon oils having boiling points of not higher than 343.degree. C. [fuel gasoline distillates (FG)+light cycle oils (LCO)] (refer to Comparative Examples described later). Further, in Japanese Patent Application Kokai Koho (Laid-open) No. 59-132939, it is disclosed that a low yield of (FG+LCO) of 66.5% results from the catalytic cracking of gas oils by using a proton-Y-type zeolite (REY) which comprises a rare-earth metal ion and were steamed at 795.degree. C.
There are other reports of catalytic cracking of petroleum hydrocarbons by using clay minerals as catalysts other than zeolites, but these methods are not better than the above-described zeolites in the point of the low yield of (FG+LCO). For example, in Proc. 5th Int. Congr. Catal., p99 (1990), it is disclosed that a low yield of (FG+LCO) of 65.5% results from the catalytic cracking of a gas oil by using a catalyst which was prepared by ion-exchanged rectorite, which is a clay mineral having a high hydrothermal resistance, with Al polycation and calcining at 400.degree. C. for 10 hours.